How will rainfall change over Hawai‘i in the future? High-resolution regional climate simulation of the Hawaiian Islands

To better understand the historical and projected future climatology and hydrologic impacts of rainfall over the Hawaiian Islands, regional climate simulations over the main Hawaiian Islands were conducted for two 10-year periods using the Weather Research and Forecasting (WRF) model in two nested domains. The first (historical) simulation was driven by the ERA-Interim global reanalysis data and observed sea surface temperature from October 2002 to September 2012. The second simulation used the Pseudo Global Warming (PGW) method to perturb historical conditions with climate change signals taken from global climate model averages. Eighty-one vertical layers were used to better resolve the trade wind inversion (TWI). Results show that the historical simulation reproduces the mean surface temperature, relative humidity, and winds with low biases and high spatial correlations. Additionally, for the historical simulation, the aggregated daily and hourly rainfall probability density functions (PDFs) and rainfall spatial-temporal distributions were well simulated, likely because WRF captured the TWI properties well. For the PGW (future) simulation, the mean rainfall increased by 2.2% in the dry season and 12.5% in the wet season; the average annual increase was 7.8%. The TWI base height decreases in future dry seasons leading to reduced rainfall on the upwind sides of high mountains. Interactions between the complex terrain and the TWI led to heterogeneous changes in patterns of future rainfall across Hawai‘i, including more than a 20% increase in extreme rates in the dry season and a ~ 10% increase during the wet season.